The basic structure of a general cellular phone will be described below. FIG. 1 is a set of schematic views showing the basic structure of foldable cellular phone 1. Specifically, FIG. 1(a) is a front elevational view of foldable cellular phone 1. FIG. 1(b) is a set of a front elevational view and a side elevational view of upper casing 2. FIG. 1(c) is a set of a front elevational view and a side elevational view of lower casing 3.
As shown in FIG. 1(a), cellular phone 1 comprises upper casing 2 positioned upwardly of hinge 4 and lower casing 3 positioned downwardly of hinge 4. Upper casing 2 and lower casing 3 have hollow box shapes, respectively. Upper casing 2 and lower casing 3 are often made of a nonmetallic material such as resin, plastic, or the like.
LCD panel 21 serving as a display means, a speaker, not shown, etc. are disposed on one surface of upper casing 2. Key pad 22 serving as an operating means, a microphone, not shown, etc. are disposed on one surface of lower casing 3. In the present specification, of the surfaces of cellular phone 1 (upper casing 2 and lower casing 2), the surface on which LCD panel 21 and key pad 22 are disposed will be referred to as a front surface, and the surface opposite to the front surface as a rear surface.
FIG. 2 is a set of views showing only the casings of cellular phone 1. FIG. 2(a) is a cross-sectional view taken along line A-A of FIG. 1(a), and FIG. 2(b) is a cross-sectional view taken along line B-B of FIG. 1(a). Break line A-A shown in FIG. 1(a) represents a straight line passing through left half hinge 5a to be described later, and break line B-B a straight light passing through central half hinge 5c. 
Referring back to FIG. 1, as shown in FIGS. 1(b) and 1(c), upper casing 2 has on its lower end substantially semicylindrical hinges (half hinges 5a, 5b) disposed respectively at left and right ends thereof. Lower casing 3 has on its upper end half hinge 5c, similar to half hinges 5a, 5b, disposed at its center. Half hinges 5a, 5b, 5c are essentially equal in contour and diameter to each other. Lower casing 3 also has indented surfaces 6a, 6b on both opposite outer sides of half hinge 5c complementarily in contour to half hinges 5a, 5b of upper casing 2. Indented surfaces 6a, 6b have such a depth and a shape that when half hinges 5a, 5b are placed on half hinges 5a, 5b, adjacent ones of half hinges 5a, 5b, 5c are superposed on each other at substantially the same height.
Half hinges 5a, 5b, 5c are referred to as “half hinges” because they have a shape corresponding to substantially one-half of a cylinder. Even if they have a shape corresponding to a portion of a cylinder (more or less than one-half thereof or a nearly prismatic shape, they will also be referred to as “half hinges” in the description which follows.
As shown in FIG. 1(a), upper casing 2 and lower casing 3 are openably and closably coupled integrally to each other by hinge assembly 4 which is made up of half hinges 5a, 5b, 5c. Hinge assembly 4 includes a coupling mechanism by which upper casing 2 and lower casing 3 are openably and closably coupled to each other. For example, hinge assembly 4 includes a cylindrical shaft extending through half hinges 5a, 5b, 5c. The coupling mechanism is omitted from illustration in FIG. 1.
FIG. 3 shows typical states in which cellular phone 1 is used. FIG. 3(a) shows an unfolded state in which the angle formed between the front surface of upper casing 2 and the front surface of lower casing 3 is maximum. FIG. 3(b) shows a folded state in which the above angle is minimum. Generally, cellular phone 1 is in the unfolded state when the person using it makes a call with the speaker of cellular phone 1 being close to the ear (during the call). On the other hand, cellular phone 1 is in the folded state when it is placed in a pocket or a bag (in the standby mode). When cellular phone 1 is in the unfolded state shown in FIG. 3(a), the angle formed between the front surfaces of upper casing 2 and lower casing 3 is about 180°. However, the angle may be smaller than 180° or greater than 180°.
As shown in FIGS. 3(a) and 3(b), upper casing 2 and lower casing 3 incorporate upper printed-circuit board 7 and lower printed-circuit board 8 respectively therein which support a radio circuit and a control circuit (not shown) thereon. Signals are sent and received between printed-circuit boards 7, 8 through a cable (a flexible cable or a coaxial cable) extending through hinge assembly 4 and connected to printed-circuit boards 7, 8. The cable is omitted from illustration.
FIG. 3(c) shows the position of antenna 9 included in cellular phone 1. Antenna 9 is disposed near hinge assembly 4 and connected to upper printed-circuit board 7. Specifically, antenna 9 is connected to a pad or a spring (power supply 13) on the lower end of upper printed-circuit board 7. Transmission power is supplied to antenna 9 through electrode supply 13 and reception power is transmitted from antenna 9 to the radio circuit.
Upper printed-circuit board 7 supports thereon a matching circuit (not shown) for adjusting the impedance of antenna 9 to match a desired impedance in upper printed-circuit board 7. The impedance matching allows radio waves to be transmitted from antenna 9 and received from antenna 9 efficiently.
During the call, the human body is present near the antenna of the cellular phone. Therefore, a portion of electromagnetic waves (electric power) radiated from the antenna is absorbed by the human body, resulting in a reduction in the antenna radiation efficiency that serves as an indicator of antenna performance. Specifically, if it is assumed that the electric power input to the antenna is represented by “Pin”, the electric power radiated into space by “Pr”, and the electric power absorbed from the radiated electric power by the human body (lost electric power) by “Ploss”, then the antenna radiation efficiency (η) during the call is expressed by the following equation (the loss of the antenna itself is ignored):η=Pr/Pin=(Pin−Ploss)/Pin=1−(Ploss/Pin)
Therefore, the lost electric power (Ploss) needs to be reduced in order to increase the antenna radiation efficiency during the call and improve the communication capability of the cellular phone.
Japanese patent No. 3838815 and Japanese patent No. 3642029 disclose a technology for placing a plate of magnetic material or metal in a space between an antenna and the human body, as a means for increasing the antenna radiation efficiency.
According to the technology disclosed in Japanese patent No. 3838815 and Japanese patent No. 3642029, an electromagnetic wave radiated from the antenna is reflected by the surface of the plate of magnetic material or metal, developing a reduced electromagnetic field in an area behind the plate. In other words, it is possible to reduce the electromagnetic wave that is applied to the human body which is positioned remotely from the antenna with the plate of magnetic material or metal interposed therebetween. As a result, the lost electric power (Ploss) is reduced. The plate of magnetic material or metal functions as a type of electromagnetic wave shield for controlling the electromagnetic field behind the plate. In the present specification, a member which performs such a function may be referred to as “control plate” or “electromagnetic field control plate”.
Many cellular phones available recently are of the foldable type, with an antenna being often installed near the hinge assembly due to packaging space limitations. For sufficiently maximizing the effect of the electromagnetic field control plate, it is necessary to cover a relatively wide area including the antenna installed area with the electromagnetic field control plate. Preferably, the electromagnetic field control plate is disposed over an area extending from a lower portion of the upper casing through the hinge assembly to an upper portion of the lower casing.
FIGS. 4(a) and 4(b) show an example in which an electromagnetic field control plate is applied to cellular phone 1 shown in FIG. 1. For increasing the antenna radiation efficiency during the call, electromagnetic field control plate 14 is preferably disposed to cover antenna 9 and a relatively wide area therearound, as described above. Consequently, electromagnetic field control plate 14 is placed so as to extend in upper casing 2 and lower casing 3 through hinge assembly 4. Specifically, through holes have shapes and sizes for allowing electromagnetic field control plate 14 to pass therethrough are defined in upper and lower casings 2, 3, and electromagnetic field control plate 14 is disposed as described above.